High temperature jet propulsion apparatus



Aug. 21, 1962 H. E. GRENOBLE HIGH TEMPERATURE JET PROPULSION APPARATUSFiled Dec. 23, 1957 ospBher/c) Prxsure (41 P 6 In van/0r Herber/ E.Grenoble,

United States Patent Ofiice 3,049,869 Patented Aug. 21, 1962 3,0493%HIGH TEMPERATURE .FET PROPULSIUN APPARATUS Herbert E. Grenoble,Amsterdam, N.Y., assignor to General Electric Company, a corporation ofNew York Filed Dec. 23, 1957, Ser. No. 704,536 3 Claims. (Cl. 6035.3)

This invention relates to high temperature gas turbines and related jetapparatus, and more particularly, to a method of operating jetpropulsion devices at extreme high temperatures where normal turbineblade and other structural materials have little mechanical strength.

It is well know that many metals and alloys connnonly used forstructural purposes suffer a substantial loss in their mechanicalproperties when their temperature is elevated. Some metals, however,exhibit much greater resistance to physical changes at high temperaturesand are referred to as refractory metals, such as molybdenum, tantalum,tungsten, and niobium. While these metals retain considerable strengthat high temperatures due to their high melting point, they have a commondisadvantage of being highly susceptible to oxidation at hightemperatures. For example, one of the limitations in operating gasturbine engines at extreme temperatures has been the lack ofsatisfactory structural materials, particularly those metals from whichthe turbine bucket or blade is manufactured. The temperature of presentgas turbines is generally limited to approximately 1700 F. for presentblade metals of the nickel-chromium type. Turbine blades manufacturedfrom the refractory metals as above mentioned may operate in thetemperature range exceeding 2000 F. provided their oxidation tendenciescan be controlled. Molybdenum, when exposed to high temperatures in airor combusted atmosphere containing some free oxygen, reacts to form atrioxide which becomes a liquid at about 1460 F. and boils at about 2335F. Tungsten is similarly limited by the existence of a volatiletrioxide, although the vapor pressure of the oxide is lower for the sametemperature conditions relative to molybdenum. Niobium and tantalum formoxides which are relatively nonvolatile and protective, but areembrittled by oxygen or nitrogen taken into solution in the metal phase.

Broadly speaking, of the refractory metals mentioned, molybdenum isleast disadvantageous because of its low density relative to tungsten,and because of its iimited solubility for oxygen and nitrogen, relativeto niobium and tantalum. While the disadvantage of oxidation is commonto all of the metals mentioned, it is perhaps most pertinent to the useof molybdenum which is otherwise highly desinable for its hightemperature strength and related characteristics. A preferred form ofthis invention includes the use of molybdenum and its alloys, and adescription of the problems associated therewith substantially sufiicesfor the remaining metals of the refractory group.

Since the trioxide of molybdenum becomes a liquid at about 1460 F. thereis little protection afforded by this trioxide against continuingoxidation at temperatures in excess of 1460 F. Actually, the oxidationof molybdenum in air may be considered to be rapid at a temperature aslow as 1100" F. Therefore, in order to use molybdenum and/ or its alloyswhere the high tempera ture, high strength characteristics are desired,it has been the practice to form a protective coating or covering on theturbine bucket or blade or other structural members exposed to hightemperatures. In this respect it is understood that the life of such acoated or covered blade is particularly limited by the life of thecovering itself, and the coating must also be capable of withstandingthe high temperatures involved. A solution to the problem may then befound in two conceptions; one, to develop a satisfactory coating orcovering which will Withstand the high temperatures, or two, to providecombustion gases which are non-oxidizing with respect to the metalemployed. This invention is, accordingly, directed to the latter of theabove-mentioned solutions.

It is therefore an object of this invention to provide an increasedoperating temperature for gas turbines or other jet propulsionapparatus.

It is another object of this invention to operate a high temperature gasturbine with turbine blades of a refractory metal including tantalum,niobium, tungsten, molybdenum, and alloys thereof.

It is a further object of this invention to remove the necessity ofcoverings or coatings for turbine blades and other parts manufactured ofa refractory metal and operating under high temperatures where suchmetals are highly susceptible to oxidation.

It is another object of this invention to provide a nonoXid-izingatmosphere adjacent turbine blades or other parts of a refractory metalemployed in gas turbines and the like apparatus.

Briefly stated, in accordance with one aspect of this invention, anon-covered or non-coated turbine bucket or other structural part of arefractory metal is employed for continuous operation in a temperaturejet propulsion cycle wherein the gas stream passing adjacent the bucketsor other related structure precludes the formation of trioxides on thebucket surfaces through a predetermined greater-than-stoichiometricburning of the fuel, for example, in the combustion chamber of a gasturbine engine. The invention is equally applicable to other jet deviceswhere certain structures exposed to a high temperature gas stream arehighly susceptible to extreme oxidation.

While this specification concludes with claims particularly pointing outand distinctly setting forth the invention, the invention will be betterunderstood when taken in connection with the following specification andthe drawings thereof, wherein:

FIG. 1 discloses an exemplary application of this invention;

FIG. 2 is a cycle illustration of this invention; and

FIG. 3 discloses a modification of the invention of FIG. 1.

FIG. 1 illustrates one preferred form of this invention as exemplary ofan application of this invention applied to a conventional turbojetengine 1. Engine 1 includes a casing 2 provided with an inlet 3 and anexhaust 4. Within the casing 2 there is positioned a compressor 5connected to a turbine Wheel 6. Turbine Wheel 6 has mounted thereonsuitable molybdenum or molybdenum alloy buckets or blades 7 which arecapable of withstanding temperatures in excess of 17030 F. in a reducingatmosphere. Fuel for the jet engine is injected into combustion chamber8 through fuel nozzle 9 and the products of combustion within thecombustion chamber 8 pass through the turbine buckets 7 for rotation ofthe turbine wheel 6 and the compressor 5. As heretofore stated,molybdenum and molybdenum alloy buckets are capable of withstandingtemperatures in excess of 1700 F. when operated in a controlledatmosphere, and accordingly the fuel-air ratio Within the combustionchamber 8 is so regulated as to burn with an excess of fuel, and thus bea gas stream which will not react with the molybdenum blade to form thevolatile trioxide of molybdenum. The fuel-rich mixture may produce asatisfactory atmosphere if only rich enough to the extent of forming thedioxide of molybdenum on the turbine buckets, the dioxide being a formof protective coating. Burning a fuel-rich mixture in a gas turbine isan uneconomical process since much of the unburned fuel is exhaustedthrough the engine without 3 benefit of the available energy therein.Wastefulness of the process may be substantially minimized byintroducing additional air into the exhaust duct 4 aft the turbine wheel6 for further burning. Such a process is quite similar to that known asre-heat operation, where, in a turbojet engine, additional fuel isinjected into the exhaust duct for further burning with a resultantincrease in thrust. One means of providing additional air in the exhaustduct of a turbojet engine is illustrated in FIG. 1 by a by-pass passageor tube 10' which permits partial extraction or flow of air from thecompressor to be introduced into the exhaust section aft the turbinewheel 6. It is therefore seen that the excess of fuel employed in theprimary combustion process as a means of atmosphere and temperaturecontrol is further utilized to augment the thrust from the jet by theafterburning or re-heat principle.

The cycle for this process is illustrated in FIG. 2 by an idealized andschematic pressure volume diagram. In FIG. 2, the line AB represents theadiabatic compression of the incoming air to the compressor, while theline BC represents the combustion of the fuel-rich mixture at a constantpressure. Line CD indicates the adiabatic expansion of the burningmixture through the turbine wheel and line DE represents the furthercombustion in the exhaust duct after the addition of air from thecompressor. The cycle concludes with line EF illustrating the adiabaticexpansion through the jet outlet. The net work per pound of gas mixturepassing through the turbine is given by the enclosed area of the curveABCDEF in FIG. 2. In comparison, it may be seen that a greater thrustper pound of mass fiow is possible from the proposed cycle than ispossible from either of two cycles, first, a low temperature cyclewithout re-heating, or second, by a low temperature cycle withre-heating by the addition of fuel in the exhaust cone. In the formercycle, the net work per pound of gas mixture is indicated by the areaenclosed by the included curve ABC and F, While in the latter cycle thenet work per pound of gas mixture is indicated by the shading enclosedby curves ABCD'E and F.

The proper fuel-air mixture to be employed in this invention in order toobtain satisfactory results must be controlled between limits. Themixture must generally be more than slightly over-rich, and sufficientto limit the gas temperature to approximately 3000 F., the maximumtemperature at which a molybdenum or molybdenum alloy bucket may beexpected to operate effectively. The extent of the richness of thismixture is also limited by the problem of satisfactory combustion of ahighly rich mixture in a rapidly moving gas stream. For example, amolybdenum turbine bucket may be effectively employed in a gas turbinewhere the temperature of the gas is maintained about 3000 F. by theaddition of fuel exceeding the stoichiometric proportion of fuel and airto the extent of about 1 /2 times the stoichiometric proportion. Underthese conditions, the molybdenum operates satisfactorily with littletendency toward rapid deterioration from oxidation.

Various types of control apparatus may be employed in connection withthis invention to maintain a constant rich mixture or a selectivelyvariable one. For example, in FIG. 1 control 11 senses the temperatureat turbine blades 7 through a suitable temperature responsive device 12.The amount of fuel entering the combustion chamber 8 from the nozzle 9is thereby controlled to maintain an equilibrium temperature conditionat the turbine blades 7. The control 11 may be further correlated with avalve 16 in the by-pass duct of the engine 1 to vary the amount of airto be added to the exhaust duct in a manner proportional to theoverly-rich mixture utilized to maintain temperature equilibriumconditions. It is to be understood that such exemplary controls may besuitably integrated to provide the proper temperature or reducingatmosphere at various throttle settings. The particular devices, and/ orelements of the control system are quite well known to those skilled inthe art, and therefore do not form any part of this invention.

Where the reheat principle of this invention is not applicable incertain gas turbine installations, one modification as disclosed in FIG.3 may be utilized with good results. Referring now to FIG. 3, there isschematically illustrated a gas turbine engine 14 which is quite similarto the gas turbine engine 1 of FIG. 1, although without the compressorby-pass ducting 10. Engine 14 is equipped with a conduit type of exhaust15 which exhausts the fuelrich exhaust stream into a secondary engine16. The secondary engine 16 may comprise a compressor 17, combustionchambers 18, and a further turbine wheel 19. The fuel-rich exhauststream from engine 14 is introduced into the combustion chambers 18 ofengine 16 where it is mixed with air from compressor 17 and, ifnecessary, additional fuel. The products of combustion pass through theturbine wheel 19 for rotation thereof. The power obtained from the shaftof engine 16 may be utilized for various purposes, or connected by meansof gearing to the shaft of engine 14 for an increased power output. Themodification as illustrated in FIG. 3 is exemplary of many arrangementsof tandem or serial positioned power plants closely related to the wellknown topping process in steam turbine practice.

The invention thus described contemplates a high temperature gas turbineor the like jet apparatus wherein the maximum temperature will belimited only by the strength of refractory metals such as those ofmolybdenum, niobium, tantalum, and tungsten, and their alloys. In a gasturbine, the combustion gases in the high temperature turbine stageswill be kept of controlled oxidation potential with respect to the metalused for the turbine buckets by burning fuel in excess of thestoichiometric ratio. As the temperature of these gases falls byextraction of energy in the form of useful work, additional air may bemixed and burned with the fuel-rich mixture, provided that atrioxidizing mixture does not result. Ultimately, an excess of air canbe added to the gas mixture when the temperature is held below 1700" F.,since structural materials of inherent oxidation resistance are known upto this value of temperature. It is to be further understood that thewhole of the products of combustion need not be a reducing atmosphere,since it is contemplated that where general or limited parts are made ofrefractory metals the controlled atmosphere need be provided onlyadjacent these parts. For example, for the blades of a turbine Wheel,fuel may be added upstream of the blades to burn and be a protectiveatmosphere while passing through the blades and thereafter to becomeoxidizing.

While other modifications of this invention and variations of apparatuswhich may be employed in the scope of the invention have not beendescribed, the invention is intended to include all such as may beembraced within the following claims.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

1. The method of increasing the operating temperature of a turbojetengine having critical operating parts including a metal from the groupconsisting of molybdenum and tungsten to the range of about 2000 F. to3000 F., said method comprising combusting a fuel-air mixture in theengine which contains about 1.5 times the stoichiometric amount of fuelso that combustion products are produced which permit dioxides ofmolybdenum and tungsten to form and preclude formation of trioxides ofthese metals, and mixing additional air with the previously combustedfuel-air mixture after ithas passed out of contact with the criticaloperating parts to effect further combustion thereof.

2. The method of operating a high temperature turbojet engine includingcompressor, combustion, turbine and exhaust sections, utilizing bladeson the turbine wheel of said engine which include a metal selected fromthe group consisting of molybdenum and tungsten, combusting a fuel-airmixture in said engine which contains excess fuel suflicient to formcombustion products preventing the formation of metal trioxides on theturbine buckets at temperatures of 2000" to 3000 F., and mixingadditional air with the previously combusted fuel-air mixture in theexhaust section of the engine to effect further combustion thereof attemperatures below 2000 F.

3. The method of operating a high temperature turbojet propulsion powerplant including first and second turbine sections in which criticaloperating parts of at least one of said sections are constructed of ametal from the group consisting of molybdenum and tungsten, said methodcomprising combusting a fuel-air mixture rich in fuel in the power plantcreating combustion products preventing the formation of trioxides ofthe metals molybdenum and tungsten at operating temperatures of fromabout 2000 F. to 3000 F and mixing additional air with the combustedfuel-air mixture intermediate the first and second turbine sections toeffect further combustion thereof and to perform additional work in thesecond turbine section.

References Cited in the file of this patent UNITED STATES PATENTS2,511,385 Udale June 13, 1950 2,611,239 Briggs Sept. 23, 1952 2,653,446Price Sept. 29, 1953 2,751,188 Rath June 19, 1956 2,766,583 SchirmerOct. 16, '1956 2,872,782 Johnson et a1. Feb. 10, 1959 FOREIGN PATENTS851,496 France Oct. 2, 1939 628,366 Great Britain Aug. 26, 1949 749,009Great Britain May 16, 1956

1. THE METHOD OF INCRESING THE OPERATING TEMPERATURE OF A TURBOJETENGINE HAVING CRITICAL OPERATING PARTS INCLUDING A METAL FROM THE GROUPCONSISTING OF MOLTBDENUM AND TUNGATEN TO THE RANGE OF ABOUT 20000*F. TO3000*F., SAID METHOD COMPRISING COMBUSTING A FUEL-AIR MIXTURE IN THEENGINE WHICH CONTAINS ABOUT 1.5 TIMES THE STOICHIOMETRIC AMOUNT OF FUELTHAT COMBUSTION PRODUCTS ARE PRODUCED WHICH PERMIT DIOXIDES OFMOLYBDENUM AND TUNGSTEN TO FORM AND PRECLUDE FORMATION OF TRIOXIDES OFTHESE METALS, AND MIXING ADDITIONAL AIR WITH THE PREVIOUSLY COMBUSTEDFUEL-AIR MIXTURE AFTER IT HAS PASSED OUT OF CONTACT WITH THE CRITICALOPERATING PARTS TO EFFECT FURTHER COMBUSTION THEREOF.